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A New Method to Produce Ni–Cr Ferroalloy Used for Stainless Steel Production

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A New Method to Produce Ni–Cr Ferroalloy Used for Stainless Steel Production High Temp. Mater. Proc. 2016; 35(7): 635–641 Pei-Xian Chen*, Shao-Jun Chu and Guo-Hua Zhang A New Method to Produce Ni–Cr Ferroalloy Used for Stainless Steel Production DOI 10.1515/htmp-2015-0054 overhead) [1], respectively. It is possible to make a profit- Received March 9, 2015; accepted June 23, 2015 able stainless steel production process by reducing nickel Abstract: A new electrosilicothermic method has been and chromium addition. Currently, there are two methods proposed in the present paper to produce Ni–Cr ferroalloy, to produce Ni–Cr stainless steel: using ferronickel instead which can be used for the production of 300 series stain- of nickel metal [2] and directly reducing chromium ore less steel. Based on this new process, the Ni–Si ferroalloy fines by carbon in a smelting reduction–kawasaki com- is first produced as the intermediate alloy, and then the bined blown converter instead of using ferrochromium desiliconization process of Ni–Si ferroalloy melt with chro- alloy [3]. mium concentrate is carried out to generate Ni–Cr ferroal- However, there are some problems in ferronickel loy. The silicon content in the Ni–Si ferroalloy produced in production process. Ferronickel is a ferroalloy typically the submerged arc furnace should be more than 15 mass% containing about 70–90% iron and 10–30% nickel. It is (for the propose of reducing dephosphorization), in order always produced by the rotary kiln-electric furnace to make sure the phosphorus content in the subsequently (RKEF) smelting process from lateritic nickel ores. But in produced Ni–Cr ferroalloy is less than 0.03 mass%. A high China, submerged arc furnace (SAF) is the most widely utilization ratio of Si and a high recovery ratio of Cr can be used furnace to produce ferronickel. Generally, to meet obtained after the desiliconization reaction between Ni–Si the requirement from the traditional stainless steel ferroalloy and chromium concentrate in the electric arc production, the ferronickel alloy is expected to be furnace (EAF)–shaking ladle (SL) process. produced with a low content of silicon so that it is expected that the ores containing low silica content or – Keywords: Ni Cr ferroalloy, stainless steel, electrosili- the reductant is less used. However, lateritic nickel ore is cothermic metallurgy, dephosphorization, desiliconization always with a high silica content so that the quantity of coke has to be controlled in a low level [4, 5], which leads Introduction to the obvious increase of resistance of furnace burden. Consequently, the operation mode with a low current and high voltage for SAF has to be required in ferronickel The growth rate of stainless steel consumption is the production process. It is very different from the tradi- highest one among all materials in the world today tional operation mode for SAF, which is used to produce (ISSF, 2009). Unfortunately, its further growth will be ferromanganese, ferrosilicon or ferrochromium alloy. restricted by the resource supply and production cost. Another problem is to increase the service life of furnace The consumption of nickel and chromium ferroalloys lining. Carbon-based lining was always adopted in the accounts for most of the cost of stainless steel produc- SAF when producing silicomanganese and ferromanga- tion: for example, 62% and 16% for nickel and chromium nese alloys. However, it cannot be used in the production in the production of stainless steel slabs (not including of ferronickel alloy because a large amount of carbon could be quickly dissolved into ferroalloy with low silicon *Corresponding author: Pei-Xian Chen, School of Metallurgical and content. Therefore, if lateritic nickel ore is used to pro- Ecological Engineering, University of Science and Technology duce ferronickel with high silicon content in the tradi- Beijing, Xueyuan Road 30, Haidian District, Beijing 10083, P.R. China, E-mail: [email protected] tionally SAF with carbon-based lining, the problems Shao-Jun Chu, School of Metallurgical and Ecological Engineering, mentioned above can be avoided. University of Science and Technology Beijing, Xueyuan Road 30, In addition, utilization of chromium concentrate is one Haidian District, Beijing 10083, P.R. China, of the most important problems in ferrochromium produc- E-mail: [email protected] tion process. Over 80% of the world’s known exploitable Guo-Hua Zhang, State Key Laboratory of Advanced Metallurgy, chromite resources are located in South Africa, in which University of Science and Technology Beijing, Xueyuan Road 30, Haidian District, Beijing 10083, P.R. China, 30% is lumpy ore and 70% is fine or friable ore [6]. E-mail: [email protected] Usually, chromium concentrate is pelletized in pelletizing 636 P.-X. Chen et al.: A New Method to Produce Ni–Cr Ferroalloy drum firstly and then charged into a sintering furnace to thermodynamics. Thereby, in the actual case, it can enhance its strength to bear the transport and impact, take place much more easily because of the high which causes the increases in the cost and the energy silicon content and low chromium content in the initial consumption for ferrochromium production. Hence, the ferroalloy melt: beneficial way for stainless steel production is to use 3½Si þ 2Cr O ð Þ þ 6CaOð Þ ¼ 4½Cr þ 3Ca SiO ð Þ chromite concentrate directly instead of ferrochromium 1% 2 3 s s 1% 2 4 l À1 alloy. ΔG ðT ¼1; 673À1; 873 KÞ¼582:343 À 0:017T kJ Á mol In view of the problems involved in the traditional ð3Þ production processes of ferronickel and ferrochromium In the new process, the ferronickel alloy containing high alloys [6], a new method is introduced to produce the silicon, which is also named as nickel–silicon ferroalloy, Ni–Cr ferroalloy in this paper, which can provide new is designed as the source of silicon. Then Cr2O3 in the sources of Ni and Cr for Ni–Cr stainless steel production chromium concentrate is reduced by nickel–silicon fer- instead of ferronickel and ferrochromium alloys. roalloy to produce Ni–Cr ferroalloy. Therefore, a process for producing 300 series stain- less steel with this method can be proposed and its flow Metallurgical fundamental diagram is shown in Figure 1. The nickel–silicon ferroal- loy is produced by sintering-SAF process and then the The new method is based on the silicothermic reduction obtained molten metal will react with chromium concen- reaction, which is always called as electrosilicothermic trates in electric arc furnace (EAF) and shaking ladle (SL) method in the industry of ferroalloys. Actually, it has to get primary stainless steel metal which can be placed been widely applied to produce ferromanganese and fer- in argon oxygen decarburization (AOD)/vacuum oxygen rochromium alloys with the low/medium carbon content decarburization (VOD) vessel for refining finally. The in the ferroalloy industry [7]. However, there is no report main aim of electrosilicothermic process presented in in the literatures about producing stainless steel with this this paper is to get the primary stainless steel which is method. The reaction principle of electrosilicothermic similar to that in the traditional stainless steel production method is always expressed as eq. (1) and it can be process. Table 1 shows the typical composition of expressed as eq. (2) when it is used to produce Ni–Cr ferroalloy: ½þSi ðÞ!MeO ½þMe ½SiO2 ð1Þ y½ þ yðÞ þ xðÞ 3 Si Fe 2 Cr2O3 chromium concentrate 3 CaO lime ¼ y½ þ ðÞx Á y 4 Cr alloy 3 CaO SiO2 slag ð2Þ If assuming that x ¼ 2, y ¼ 1 in eq. (2), the change of standard Gibbs free energy can be calculated as shown in eq. (3) by the data from literatures [8, 9]. In the temperature range of 1,673–1,873 K, the change of standard Gibbs free energy is –611.186 to –614.634 kJ · mol−1,which indicates that reaction (3) can spontaneously happen Figure 1: Flow diagram of electrosilicothermic process for 300 series in the standard state from the view point of stainless steel production. Table 1: Chemical compositions of 300 series stainless steel in duplex and triplex processes. Process Name C (%) P (%) S (%) Si (%) Mn (%) Cr (%) Ni (%) Duplex Primary steel < <. <. <.. . Refined steel <. <. <. Triplex Primary steel <. <. – <.. . Refined steel <. <. – .... P.-X. Chen et al.: A New Method to Produce Ni–Cr Ferroalloy 637 300 series stainless steel required in the traditionally extracted from lateritic nickel ore in SAF was taken from duplex and triplex stainless steel production processes. one plant in China. As shown in Table 1, except for Fe, Cr and Ni, attention should also be paid on phosphorus and silicon in stain- less steel production. It means that if phosphors and Process silicon can be controlled in the new electrosilicothermic process, it is possible that the primary 300 series stain- In the de-P experiment, the nickel–silicon ferroalloy with less steel is produced by this new process. about 0.04–0.06 mass% P and about 5–30 mass% Si was smelted in MgO crucible by using ferronickel alloy, metal- lic silicon and ferrophosphorus as the raw materials. Then alloy samples were taken by quartz tube before Experimental adding the de-P agent. After reacting for 10 min, another alloy sample was taken again. The weight ratio of de-P In order to determine whether it is possible to produce agent to metal was about 1:10. Two kinds of de-P agent – appropriate Ni Cr ferroalloy by the new method or not, were used: the one for precipitation de-P contains about some technological experiments have been done in a 50% calcium-silicon alloy, 25% lime and 25% fluorite; the pilot-scale plant, including dephosphorization (de-P) other one for interface de-P contains about 60% lime and – and desiliconization (de-Si) experiments in nickel silicon 40% fluorite.
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